(function() {

   /**
    * Changes value from one to another within certain period of time, invoking callbacks as value is being changed.
    * @memberOf fabric.util
    * @param {Object} [options] Animation options
    * @param {Function} [options.onChange] Callback; invoked on every value change
    * @param {Function} [options.onComplete] Callback; invoked when value change is completed
    * @param {Number} [options.startValue=0] Starting value
    * @param {Number} [options.endValue=100] Ending value
    * @param {Number} [options.byValue=100] Value to modify the property by
    * @param {Function} [options.easing] Easing function
    * @param {Number} [options.duration=500] Duration of change
    */
  function animate(options) {

    requestAnimFrame(function(timestamp) {
      options || (options = { });

      var start = timestamp || +new Date(),
          duration = options.duration || 500,
          finish = start + duration, time,
          onChange = options.onChange || function() { },
          abort = options.abort || function() { return false; },
          easing = options.easing || function(t, b, c, d) {return -c * Math.cos(t/d * (Math.PI/2)) + c + b;},
          startValue = 'startValue' in options ? options.startValue : 0,
          endValue = 'endValue' in options ? options.endValue : 100,
          byValue = options.byValue || endValue - startValue;

      options.onStart && options.onStart();

      (function tick(ticktime) {
        time = ticktime || +new Date();
        var currentTime = time > finish ? duration : (time - start);
        if (abort()) {
          options.onComplete && options.onComplete();
          return;
        }
        onChange(easing(currentTime, startValue, byValue, duration));
        if (time > finish) {
          options.onComplete && options.onComplete();
          return;
        }
        requestAnimFrame(tick);
      })(start);
    });

  }

  var _requestAnimFrame = fabric.window.requestAnimationFrame       ||
                          fabric.window.webkitRequestAnimationFrame ||
                          fabric.window.mozRequestAnimationFrame    ||
                          fabric.window.oRequestAnimationFrame      ||
                          fabric.window.msRequestAnimationFrame     ||
                          function(callback) {
                            fabric.window.setTimeout(callback, 1000 / 60);
                          };
  /**
    * requestAnimationFrame polyfill based on http://paulirish.com/2011/requestanimationframe-for-smart-animating/
    * In order to get a precise start time, `requestAnimFrame` should be called as an entry into the method
    * @memberOf fabric.util
    * @param {Function} callback Callback to invoke
    * @param {DOMElement} element optional Element to associate with animation
    */
  function requestAnimFrame() {
    return _requestAnimFrame.apply(fabric.window, arguments);
  }

  fabric.util.animate = animate;
  fabric.util.requestAnimFrame = requestAnimFrame;

})();